The present invention relates to a method and apparatus for transpiration cooling of a portion of the vehicle and in particular to cooling of a leading edge having a low radius of curvature.
High velocity vehicles such as air-breathing hypersonic vehicles typically must be provided both with a system for minimizing drag and for cooling portions of the vehicle, to accommodate heating rates. Accommodating both these requirements is particularly difficult for vehicles intended for sustained high velocity flight and/or orbital insertion, as opposed to reentry vehicles. Requirements during sustained flight and/or orbital insertion include operation over extended periods of time in an extremely hostile thermal environment. Stagnation temperatures for vehicles approaching orbital velocities can exceed 10,000.degree. K. Attempts to achieve high net thrust (i.e., thrust minus drag), lead to designs in which a vehicle may be required to operate in a high dynamic pressure (high thrust) environment with sharp leading edges (to provide low drag). As discussed below, this combination of high temperature, high pressure and small leading edge radius is, generally, considered antagonistic to attempts to cool the vehicles since sharp leading edges are thought to produce extremely high local heat fluxes. In contrast, the thermal problem facing reentry vehicles is different because in reentry vehicles, high drag is desirable or at least acceptable. This allows a large leading edge radius to be used, accommodating installation of cooling systems. Furthermore, because of the typically short exposure of reentry vehicles to high temperature environments, the use of transient cooling systems, such as a heat sink or ablative techniques is feasible.
As noted above, in some respects the requirements of providing low drag and providing cooling are antagonistic. As the leading edge radius of curvature is reduced, the total heat load on the vehicle is reduced, as well as the drag. Unfortunately, the peak heat flux (as opposed to the total heat load) increases as the radius of that portion decreases. In general, the peak heat flux on the leading edge (Q.sub.LE) is inversely proportional to the square root of the radius of curvature of the leading edge (R.sub.LE), i.e., Q.sub.LE .varies. (R.sub.LE).sup.-1/2.
One type of cooling system used for portions of vehicles is a regenerative system, in which a heat transfer fluid (typically the fuel) contacts the interior surface of a vehicle skin, absorbing heat therefrom, and flows to the engine. However, as the leading edge radius of curvature is reduced, it becomes more difficult to install a regenerative circuit because of the small radius and slender section available for flow of the heat transfer fluid. Additionally, because the regenerative system depends on heat transfer through the skin, the regenerative or "back side convective" cooling system is limited by the thermal resistance of the structural material of the vehicle.
Another cooling system which has been used is transpiration cooling. In a transpiration cooling system, there is no return flow of the transpirant to the source. In transpiration cooling, fluid is conveyed to the interior surface of the vehicle skin and permitted to flow through perforations or pores through the vehicle skin. At a low rate of flow (or "blowing"), the transpirant fluid dilutes the hot boundary layer, reducing the driving enthalpy and (normally) the heat flux. This reduction in heat flux is referred to as "partial blockage". At higher blowing rates, the hot boundary layer is pushed completely away from the surface. Under these conditions, the surfaces are exposed only to the coolant temperature and the heat flux (ignoring radiation) is reduced to zero (providing full blockage). However, operating a transpiration cooling system at a blowing rate sufficient to continuously provide full blockage produces certain undesirable effects. Because the transpirant flow rate is high, there is a large consumption of transpirant and the weight of the large volume of transpirant which must be carried increases the size of the fuel tank needed for the vehicle. Further, the transpiration system produces an amount of drag which increases as the blowing rate increases. Furthermore, when the leading edge being cooled is in the vicinity of the vehicle engine (e.g., the leading edge of the engine cowl) a high blowing rate leads to high fuel ingestion into the engine inlet.
The above described cooling problems are compounded in the case of portions of the vehicle which may be subjected to shock interactions. An example is a hypersonic engine cowl. When the vehicle passes through its inlet design mach number, the vehicle fore body shock(s) interact with the cowl shock. This interaction from the two shocks (referred to as "shock-on-lip") produces a supersonic jet that sweeps across the cowl leading edge. Heat flux increases of 20 times have been reported to result from impingement of this jet on the surface of the cowl. Accordingly, there is a need for a cooling system which can accommodate heat flux increases from a supersonic jet without providing an unacceptable amount of coolant consumption, drag, or fuel ingestion, over the duration of a flight or mission.